Estrogen receptor-1 (ER) and progesterone receptor (PR) are ligand-induced transcription factors. Accessory proteins, termed coactivators modulate the transcriptional activation of these factors. It has been shown that coactivators enhance transcription of the target genes and play important roles in diverse pathological processes, such as cancers, inherited genetic diseases, metabolic disorders, and inflammation. Earlier work by our laboratory identified E6-associated protein (E6-AP) as a coactivator of steroid hormone receptors. In an attempt to identify E6-AP-interacting protein(s), we have cloned WW-domain binding protein-2 (WBP-2) and have shown that WBP-2 selectively coactivates the transactivation functions of ER and PR. WBP-2 is a novel polyproline rich (PPXY) motif containing protein. The PPXY motif of WBP-2 has been shown to be involved in transcriptional activation pathways. The PPXY motif interacts with WW-domain containing proteins and form PPXY motif and WW-domain (PPXY-WW) complex and mediates transcription stimulation. The PPXY-WW module has gained prominence in the last decade and many new roles of the PPXY-WW complex in the nucleus, cellular pathways, physiological and pathological processes are just emerging. WW-domains have been observed on several key regulatory proteins like yes associated protein (YAP1) and WW-domain- containing oxidoreductase (WWOX1). YAP1 has been shown to act as transcriptional coactivator for several transcription factors and has also been classified as an oncoprotein, whereas, WWOX1 has been identified as a tumor suppressor. Our preliminary data suggest that oncogenic YAP1 in conjunction with WBP-2 enhances ER and PR signaling. This increased in ER and PR signaling is attenuated by the tumor suppressor WWOX1 but the exact mechanism by which these proteins regulate transcription, cell function, growth and tumorigenesis is largely unknown. In order to understand the mechanism of action of these proteins and their physiological and pathological relevance, we hypothesize that the coactivation functions of oncoprotein YAP1 is dependent on WBP-2 where the WBP-2:YAP1 complex enhances ER and PR transactivation functions resulting in breast tumorigenesis. We further hypothesize that WWOX1 acts as a tumor suppressor by physically blocking the formation of this complex (WBP-2:YAP1) resulting in the attenuation of their coactivation and oncogenic functions. This hypothesis will be test by the following specific aims: A) To determine the role of WW-domain containing proteins, YAP1 and WWOX1 in ER and PR-mediated gene transcription; B) To decipher the mechanism by which WBP-2, YAP1 and WWOX1 regulate ER and PR transactivation functions; and C) To determine the role of WBP-2, YAP1 and WWOX1 in breast tumorigenesis. PUBLIC HEALTH RELEVANCE: Estrogen and progesterone receptors are key regulatory proteins that affect many physiological and pathological processes such as reproduction, metabolism and cancer etc. Coactivators are proteins that regulate the functions of these receptor proteins and are also key players in the development of different types of cancers. Given these facts, the understanding of coactivator biology and their mechanism of action will be useful in developing new therapies.